1. Field of the Invention
This invention relates to a new strain belonging to genus Thermus which produces a new .beta.-galactosidase, a new .beta.-galactosidase and a process for producing the same.
2. Prior Art
.beta.-galactosidase is an enzyme which decomposes lactose into glucose and galactose, and is used for the production of foods such as lactose-hydrolyzed milk, lactose-hydrolyzed solution, etc. The .beta.-galactosidase widely exists in from animals and plants to microorganisms such as fungus, yeast, bacterium, etc., and enzymes which are now used in the food industry are relatively less thermostable .beta.-galactosidases mainly derived from fungus or yeast. These .beta.-galactosidases are disclosed in the official gazette of Jap. Pat. Publin. No 24094/1978 (Precedent 1) and the official gazette of Jap. Pat. Appln. Laid open (Kokai) No. 44287/1977 (Precedent 2).
On the other hand, thermostable .beta.-galactosidases are disclosed in Biotechnology and Bioengineering [vol. 26, p. 1141 (1984)](Precedent 3), Journal of Applied Microbiology [vol. 2, p. 390 (1980)](Precedent 4), Canadian Journal of Microbiology [vol. 22, p. 817 (817)](Precedent 5), the official gazette of Jap. Pat. Appln. Laid-open (Kokai) No. 154991/1981 (Precedent 6) and Journal of Bacteriology [vol. 110. P. 691 (1972)](Precedent 7). However, these conventional .beta.-galactosidases all have one or more disadvantages such as low thermostability, narrow range of optimum pH and the inhibition of enzymatic action by a reaction product, as will be described later (see Table 2). Thus, the fact is that the conventional .beta.-galactosidases are not put into practical use.
3. Problems to be Solved by the Invention
As described above, .beta.-galactosidases now used for the production of foods on an industrial scale are relatively less thermostable enzymes, so that the lactose decomposition treatment is carried out generally at 55.degree. C. or less. In addition, milk or lactose solution as a raw material is a preferable nutrition source for bacteria. As the result, the putrefaction owing to the saprophyte contamination during the treatment is a serious problem in the food production. Furthermore, an immobilized .beta.-galactosidase results in its being insufficiently washed and sterilized after the completion of the production, as is also a serious problem in the food production. In order to solve these problems, the treatment of an object by using a thermostable .beta.-galactosidase or the washing and the sterilization at a high temperature at which saparophytes are difficult to proliferate is desired, and the development of industrially applicable thermostable .beta.-galactosidase is strongly demanded.
In order to meet such a demand, the search for the foregoing thermostable galactosidase had been attempted, and various thermostable .beta.-galactosidases were found. However, there is no conventionally obtained thermostable .beta.-galactosidase which has all of enzymological properties described in the following Items (1) to (3) required for the industrial use. That is, there is no thermostable .beta.-galactosidase which attains the level of practical application:
(1) Having sufficient thermostability:
Because the thermocoagulation of milk or defatted milk takes place at 78.degree. C. to 80.degree. C., it is desired that the enzymatic treatment is carried out at a temperature as higher as possible below said temperature. Thus, the enzyme is required to have a sufficient thermostability in the neighborhood of 70.degree. C. to 75.degree. C.
(2) Having an optimum pH in the range of neutrality to acidity:
As raw materials to be treated with .beta.-galactosidase, milk, defatted milk, cheese whey, lactose solution, etc. are assumed. These are raw materials in the neutral (ph 6.5) to acidic (pH 4.5) range, and thus are required to have sufficient enzymatic activities in said range.
(3) Low inhibition of enzymatic activity by reaction products:
It is required that the lowering of the enzymatic activity by galactose and glucose as reaction products is small when .beta.-galactosidase acts on lactose.
With an intention of creating a .beta.-galactosidase having the properties described above, the present inventors tried to purely isolate a microorganism producing such a .beta.-galactosidase from nature. As a result of it, they found that a microorganism belonging to genus Thermus as an extreme thermophille produced the .beta.-galactosidase having the properties described above from among a large number of microorganisms producing .beta.-galactosidase, and have completed the present invention.